In spectrophotometry, radiation from a source passes through a sample cell to a photodetector which measures the amount of radiation absorbed by the sample fluid in the cell. The output of the detector is a measure of absorbence of a particular wavelength of radiation. The quantitative presence of certain materials in the sample is identified by particular wavelengths characteristically absorbed by the materials. An important use of spectrophotometric detectors is in chromatography wherein the components of a chromatographic column are separated in a column and the radiation absorbence of the separated components are then measured by a spectrophotometric detector.
In such detectors, radiation transparent optical windows allow radiation from the source to pass through the cell to the detector. In a common spectrophotometric detector, radiation passes through an entrance window, through the cell in a direction parallel to the flow of sample fluid through the cell, and through an exit window to the detector. Flat windows or plano convex lenses typically have been used. U.S. Pat. No. 4,192,614-deMey, et al shows a detector assembly with flat windows at the entrance and exit openings in the cell. A lens focuses the radiation in a pattern which converges in the cell.
Another type of commonly used detector has divergent optics with sample fluid flow across a substantially planar radiation field in the cell. Such crossflow cells are typified by the Milton Roy LDC microcell used in conjunction with the LDC Model 12O4D spectroMonitor detector. Crossflow cells permit close coupling of the cell to the outlet end of a chromatographic column, but these cells are limited in available pathlength. Parallel flow cells, where the light rays travel substantially parallel to the direction of fluid flow in the cell, are less limited in available pathlength of light rays in the cell and can provide better sensitivity for sample detection. Crossing of the light rays in the parallel flow cell makes it less sensitive changes in flow rate because the light rays do not graze the lateral walls of the cell where the transverse temperature gradients are largest.
The flat or plano convex windows used in prior art detectors are sealed to the cell body with gaskets having gaps for the passage of radiation through the cell. These gaps contribute significantly to peak spreading in present commercially used detectors at the flow rates encountered in microbore high pressure liquid chromatography (HPLC). Furthermore, the cell body must have a smooth, flat surface for good gasket seals. This has been a perpetual manufacturing problem.
The use of plastic materials in the construction of cell bodies has advantages. Lateral thermal gradients in parallel flow cells make light absorption detectors sensitive to changes in solvent flow rate because of the Schlieren refraction of the light rays as they pass through the cell. One solution to this problem is to construct the cell wall of a thermally insulating material such as plastic so that heat does not flow laterally in the cell, and the lateral thermal gradients are greatly diminished. Cells have been constructed of plastic in the past, but creep of the plastic under the flat cell window surfaces due to sealing pressure often causes the windows to crack. The cracked windows leave indentations in the plastic which hinder further use of the cell body with new windows.
Spherical balls have long been used as optical focusing elements. See "MIRRORS, PRISMS AND LENSES" by James P. S. Southall, Dover Publications, 1964, p. 396. Recently, precision balls have been used for fiber optic coupling. See Product Design and Development, March, 1984, p. 3.
Spheres have not been used to form the cell windows in spectrophotometric detectors, possibly because of the short focal lengths of the spheres, and a perceived difficulty in coupling them to the rest of the optics.
It is an object of the present invention to provide a direct glass to metal seal for a spectrophotometric cell with a radiation transparent spherical ball pressed into the optical opening in the cell body.
It is another object of the present invention to produce a convergent ray path in the sample cell with focusing by a spherical ball as the window for the cell.
It is another object of the present invention to provide a window for a detector cell which makes the use of a plastic cell body practical.
It is another object of the present invention to provide a detector cell assembly which is particularly suitable for use in microbore HPLC wherein a chromatographic column can be positioned very closely to the detector cell.